Ariane currently has a better reliability record than SpaceX. Cost isn't everything - saving $100M on a $10B satellite like the JWST (which will use an Ariane) doesn't do you any good if the $10B evaporates in a launch failure.

As of now, Ariane has 82 successful flights and a partial failure. If the plans of SpaceX pan out, they will have about 60 successful flights by the end of 2018, with no partial failures, so it becomes harder to compare. They could even reach parity with Ariane by the time JWST launches.

JWST is a payload nobody wants to be responsible for blowing up, I don't think SpaceX necessarily wants it on their roster. Even a 1% chance of failure could be too much for what would be a public confidence disaster, likely to cut off significant govt funding and hamper long term prospects.

SpaceX has lost two payloads, CRS-7 and Amos-6. Ariane's record still looks pretty good, even for SpaceX fans like myself.

Ariane 5 had two failures and 3 partial failures, I'm only referring the the most recent streak of successes which Ariane just broke, and comparing with the same from SpaceX.

For those curious: CRS-7 was a resupply mission for the ISS, had a bunch of miscellaneous cargo on it (a bunch of food, experiments, etc.) Nothing too important as far as I can tell. 99% American/European/Japanese supplies. And a single thing for the Russians: a wrench :)

Amos-6 was an Israeli communications satellite that would've provided satellite internet over Europe and Africa.

So what you're saying is that the failed mission had a wrench thrown into it by the Russians? :)

Amos-6 was an interesting one only because it blew up in the ground days before launch. Ultimately, it was SpaceX's responsibility, but does it count as a "launch failure"?